VCS is an API for abstracting access to all version control systems from Perl code.
This is achieved in a similar fashion to the DBI suite of modules.
There are "container" classes,
VCS::Dir,
VCS::File,
and VCS::Version,
and "implementation" classes,
such as VCS::Cvs::Dir,
VCS::Cvs::File,
and VCS::Cvs::Version,
which are subclasses of their respective "container" classes.

The container classes are instantiated with URLs.
There is a URL scheme for entities under version control.
The format is as follows:

The "query" part is ignored for now. The path must be an absolute path, meaningful to the given class. The class is an implementation class, such as VCS::Cvs.

The "container" classes work as follows: when the new method of a container class is called, it will parse the given URL, using the VCS->parse_url method. It will then call the new of the implementation's appropriate container subclass, and return the result. For example,

For the purposes of concretely illustrating the layout of an implementation suppose that there is a need for a VCS::Imp or $Imp implementation.

An implementation class is recognised as follows: its name starts with VCS::, and require "VCS/Imp.pm" will load the appropriate implementation classes corresponding to the container classes - VCS::Imp::Dir, VCS::Imp::File and VCS::Imp::Version..

Hence in MANIFEST, or Unix file specificaton, syntax the Imp implementation ought to be composed of at least these files:

VCS/Imp.pm
VCS/Imp/Dir.pm
VCS/Imp/File.pm
VCS/Imp/Version.pm

In addition to the usual module distribution files such as MANIFEST, Makefile.PL, etc.

Implementation classes must include documentation for their special requirements, such as mandatory environment variables. See VCS::Cvs for an example.

If a method, or an argument to a method makes no sense for a particular implementation, then the implementation may ignore it, but must do so quietly.

The modules that make up a VCS implementation might act as wrappers around a command language interpreter (CLI or shell) command, or might directly look at files or talk over the network. It is recommended that before implementing a VCS::*::* class that implements direct access, the developer first implement a version that uses the CLI tools, and writes appropriate tests for it. Doing this will ensure that the direct-access version, which passes the tests, will be correct.

If you prefer there is a pod stub in Imp_pod.tpl that you could catenate onto the tail of your VCS/$Imp.pm file, making sure to edit the template with s/Imp/$Your_VCS_Name/g. For simple version control system APIs (i.e. with a CLI interface and no need for XS, C, or header file worries) an easy way to get started would be to use h2xs like so:

h2xs -AXf -n VCS::Imp

then add a VCS/Imp/VCS directory and put appropriate File.pm, Dir.pm and Version.pm modules in it (add them the the MANIFEST as well). If calling in to your version control system requires XS then omit the -X and -f switches and specify the header file name.

You might also be interested in looking at the other implementations for tips on how to map your version control systems' CLI commands to VCS::* methods. For parsing details it can prove helpful to see an example of the output of some of the other version control systems. (e.g. the output of the $LOG_CMD in typical usage).